This invention relates to piezoelectric substances for use in ultrasonic vibrators, ceramic filters, etc., and particularly in ultrasonic probes for ultrasonic-diagnostic scanners, etc.
Heretofore, the so-called PZT piezoelectric ceramics based on lead zirconate titanate (PbZrO.sub.3 -PbTiO.sub.3) as the main component have been utilized as such kind of piezoelectric ceramics, but when the frequency exceeds 5 MHz, for example, in the ultrasonic probes, the PZT piezoelectric ceramics have a problem in fabricating array transducers, because the PZT ceramics are excited not only in the necessary thickness vibration for generation of ultrasonic waves, but also strongly in the lateral vibration. To drive the vibrator as an ultrasonic probe at the predetermined frequency without any effect of unwanted lateral vibration, the shape of vibrator is restricted, where the thickness of vibrator is determined by the frequency, and the width of vibrator is so determined that no effect of unwanted vibration may be brought about. Suppose the thickness and width of a vibrator are t and w, respectively, in the PZT ceramics, it is known that the vibrator is practically used in the range of w/t&lt;1 and the highest efficiency can be obtained in w/t=0.5-0.8. To obtain an efficient probe without any influence of the unwanted vibration, a vibrator corresponding to one electrode must be divided into many arrays that are to be driven by one electrode.
The conventional system for dividing an ultrasonic probe vibrator for use at 3.5 MHz with a PZT ceramic into many arrays will be described below, referring to FIG. 1. An array itself can function as a vibrator and thus may be hereinafter referred to also as a vibrator in the specification. Thickness t of vibrator 1 is determined to be 0.4 mm so that the frequency of vibrator may be 3.5 MHz. Since w/t=0.6 the optimum width w will be 0.27 in this case. In the probe, a set of three arrays is fixed to one electrode 2 only with such complicated structure that the maximum efficiency can be obtained without any unwanted vibration. In an actual fabrication, electrodes 2 are provided on a vibrator having a large area, and fixed to a backing member 3, and then only the vibrator is divided into arrays by cutting so that three arrays can be provided on each electrode.
The ultrasonic-diagnostic scanner is widely utilized in diagnosis of body organs, etc., because it can display a working image in an actual time with easy manipulation and distinguished safety. Recently, high performance diagnosis not only of body organs but also body surface parts, for example, skin, eye, tissue, etc. has been demanded by further improving the sensitivity or resolving power of a scanner, thereby obtaining improved images. As a step for obtaining improved images, it would be possible to increase the frequency of ultrasonic wave to increase the resolving power. For example, in fabrication of a probe for use at 5 MHz from the PZT ceramics, the width of vibrator is as small as about 0.2 mm, and thus it is quite difficult to divide the vibrator into arrays by cutting to obtain higher frequency arrays.
Recently, PbTiO.sub.3 piezoelectric ceramics having a smaller electromechanical coupling factor of unwanted lateral vibration mode kp as a cause for the restriction than the electromechanical coupling factor of necessary thickness vibration mode kt have come to public attention as materials for a high frequency ultrasonic probe. So far, ceramics of (Pb.sub.1 -(3/2xSm.sub.x)(Ti.sub.1-y Mn.sub.y)O.sub.3 system and (Pb.sub.1-x Ca.sub.x)[(Co.sub.1/2 W.sub.1/2).sub.y Ti.sub.1-y ]O.sub.3 +M.sub.n O system with kp.perspectiveto.0.05 and kt.perspectiveto.0.50 (kt/kp.perspectiveto.10) have been developed and their utility has been found remarkable because the lead titanate ceramics have peculiar characteristic suitable for increasing the frequency as given below:
(1) extremely larger electromechanical coupling factor of thickness vibration mode kt than that of lateral vibration mode kp (i.e. extremely large electromechanical anisotropy of electromechanical coupling factor), for example, kt/kp=5-10 and kt=0.45-0.50, and
(2) low dielectric constant .epsilon..sub.33.sup.T, for example .epsilon..sub.33.sup.T .perspectiveto.200. Particularly since kp of the lead titanate ceramics is smaller at least in one order than that of the PZT ceramics as shown in the above (1), the lead titanate ceramics are hardly excited in lateral vibration. That is, the influence of lateral vibration remains low with increasing width of vibrator, and thus the width of vibrator can be much less restricted and vibrators with w/t=0.5-2.0 can be used.
FIG. 2 also shows a configuration of divided arrays in a high frequency probe with the conventional lead titanate ceramics, where the same electrodes 2 and backing memebers 3 as used in the PZT ceramics can be used. One vibrator 4 or lead titanate ceramics can be provided at one electrode 2 and its response to pulse wave form has been found substantially equivalent to that of the conventional vibrator.
However, since even the lead titanate ceramics still have such drawbacks that (1) w/t is restricted and (2) kt is as small as 45-50% and thus the sensitivity is low, it has been demanded in the field of ultrasonic probes, etc. to use materials having a larger electromechanical coupling factor ratio kt/kp and, if possible, a larger kt in fabricating a high frequency vibrator.
The following references are cited to show the state of the art; (i) Japanese Patent Application Kokai (Laid-open) No. 55-46620, (ii) Japanese Patent Application Kokai (Laid-Open) No. 55-67370, (iii) Japanese Patent Publication No. 44-26508 and Japanese Patent Publication No. 46-16632.